Polymerization of conjugated dienes with a homogeneous Ziegler–Natta catalytic system based on a carbon-bridged bis(phenolate) yttrium alkyl complex

Abstract

A novel carbon-bridged bis(phenolate) yttrium alkyl complex [(EDBP)Y(CH₂SiMe₃)(THF)₃] (1) was synthesized by a reaction of 2,2′-ethylidene-bis(4,6-di-tert-butyl-phenol) (EDBPH₂) and Y(CH₂SiMe₃)₃. The structure of complex 1 was determined by X-ray crystallography. Homo- and copolymerization of isoprene and 1,3-butadiene was carried out and catalyzed by the 1/MAO/triisobutylaluminum (TIBA) system. The effects of TIBA/MAO and the total [Al]/[1] molar ratio on the activity and regioselectivity of polymerization, kinetics and controllable characteristics of the polymerization system were also investigated.

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Introduction

Rare earth based catalysts have played a very important role in polymerization chemistry especially in polymerization of 1,3-butadiene and isoprene.¹ These catalysts afford polymers in good yields and with excellent properties, which are suitable for tires and other elastic materials. The conventional Ziegler–Natta type rare earth catalysts are easy to prepare, economic, thermally stable and are not very moisture and oxygen sensitive. However, because of the heterogeneity of these catalyst systems and multiple natures of active species, the molecular weight and molecular weight distribution of the resulting polymers are not well controlled.

Recently, a variety of homogeneous rare earth based catalytic systems have been developed for polymerization of 1,3-diene monomers using methylaluminoxane (MAO)² or modified methylaluminoxane (MMAO)³ as co-catalyst. The introduction of triisobutylaluminum (TIBA) into MAO activated catalytic systems has been proved to facilitate the formation and stability of active species of transition metals for polymerization.⁴ Dong and co-workers⁵ found that the activity of Nd(vers)₃/MMAO for isoprene polymerization significantly increased when Al(i-Bu)₂H partially substituted MMAO. However, to our best knowledge, no research work about the application of combination of TIBA and MAO as co-catalysts on rare earth catalytic system for the polymerization of 1,3-dienes was reported.

On the other hand, many alkali metal,⁶ poor metal,⁷ transition metal⁸ and rare earth metal⁹ based complexes supported by carbon-bridged bis(phenolate) ligands were synthesized and used to catalyse ring-opening polymerization of cyclic esters, whereas polymerization of 1,3-diene¹⁰ catalyzed by these kind of complexes is rarely reported. Carbon-bridged bis(phenol)s such as 2,2′-ethylidene-bis(4,6-ter-di-tert-butylphenol) (EDBPH₂) could be used as antioxidants for stabilizing polymers against degradation and/or discoloration.¹¹ If it could be used as a ligand for polymerization catalysts, the residue of carbon-bridged bis(phenol)s in products could serve as stabilizers directly. In this paper, we synthesized an alkyl yttrium complex supported by a carbon-bridged bis(phenolate) ligand for the first time. Activated by TIBA modified MAO, the alkyl yttrium complex was employed to catalyse a polymerization of isoprene and 1,3-butadiene as well as their copolymerization.

Results and discussion

Synthesis and characterization of alkyl yttrium complex

The carbon-bridged bis(phenolate) alkyl yttrium complex [(EDBP)Y(CH₂SiMe₃)(THF)₃] (1) was synthesized by the reaction of EDBPH₂ and Y(CH₂SiMe₃)₃ in 1 : 1 molar ratio as shown in Scheme 1. Crystals of complex 1 suitable for X-ray diffraction study were grown by cooling the hexane solution to −30 °C for several days.

Scheme 1 Synthesis of complex 1.

The molecular structural diagram of complex 1 is shown in Fig. 1 with selected bond lengths and bond angles. Complex 1 is monomeric in solid state. The yttrium atom is six-coordinated with two oxygen atoms from a bis(phenolate) ligand, a trimethylsilylmethyl group and three THF molecules, adopting a distorted octahedral geometry. The coordination structure of complex 1 is similar to that of [La(1,1-(2-OC₆H₂-tBu₂-3,5)₂)(THF)₃].¹² The yttrium to carbon distance is 2.4328(44) Å in complex 1, comparable with other yttrium complexes with trimethylsilylmethyl group.¹³ The two (Ar)O–Y bond lengths are 2.1172(26) and 2.1107(26) Å which are smaller than the discrete ion-pair carbon bridged bis(phenolate) yttrium complex.^9c The bite angle of bis(phenolate) ligand in complex 1 is 97.93(10)°, larger than that of directly linked di(phenolate)lanthanide alkyl complex¹² but smaller than that of sulfur or amine bridged bis(phenolate) lanthanide alkyl complexes.^13c,13g,14

Fig. 1 X-ray crystal structure of complex 1 with 30% probability of thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Y–C(34) 2.4328(44), Y–O(1) 2.1172(26), Y–O(2) 2.1107(26), Y–O(3) 2.4550(31), Y–O(4) 2.4500(31), Y–O(5) 2.4485(29); angles (°): O(1)–Y–O(2) 97.93(10), O(1)–Y–O(5) 162.60(11), O(2)–Y–O(4) 170.15(12), O(3)–Y–C(34) 164.44(15).

Polymerization of isoprene and 1,3-butadiene

The 1/Al binary system with MAO and/or TIBA as co-catalyst was used for homopolymerization of isoprene (IP), and the results are listed in Table 1. When MAO was used as co-catalyst, the catalyst system show low catalytic activity (26.4 kg PIP per mol [Y] per h), and the cis-1,4 content of PIP is relatively high (90.3%). The 1/TIBA binary system is inactive to isoprene polymerization despite the [Al]/[1] molar ratio varying from 10 to 200. It was reported that the addition of TIBA would increase the catalytic activity of several transition metal complex/MAO system.⁴ Here similar phenomena were confirmed and the influence of increments of TIBA on isoprene polymerization with 1/MAO/TIBA system was investigated. While the overall [Al_Tot]/[1] was kept constant, the content of TIBA (TIBA/Al_Tot) was increased gradually from 0 to 100% as shown in Table 1. It is obvious that the TIBA/Al_Tot influenced the catalytic activity and the selectivity of microstructures. When TIBA/Al_Tot increased from 10% to 15%, the activity dramatically increased from 109 to 218 kg PIP per mol [Y] per h. As TIBA/Al_Tot continued to increase from 30% to 85%, the activity decreased gradually from 115.5 to 56.1 kg PIP per mol [Y] per h. It can be concluded that using the combination of MAO and TIBA as co-catalysts can efficiently enhance the catalytic activity of isoprene polymerization. Meanwhile, the selectivity of microstructures changed with the addition of TIBA. Content of vinyl-3,4 units increased before TIBA/Al_Tot is 50% and then decreased. That is, the vinyl-3,4 content could be directly controlled by an adjustment of TIBA/Al_Tot.

Table 1 Polymerization of isoprene with different TIBA contents^a

Entry	TIBA/AlTot^b (%)	Conv. (%)	Activity (kg PIP per mol [Y] per h)	Mn^c (10^4)	PDI^c	cis-1,4^d (%)	Vinyl-3,4^d (%)
a Conditions: [IP]/[AlTot]/[1] = 800/200/1, aging at 50 °C for 1 h, polymerization in toluene, 50 °C for 15 min, [IP] = 5 M.b [AlTot] = [MAO] + [TIBA].c Determined by GPC (THF, PS calibration).d The microstructure was determined by FTIR.
1	0	12	26.4	7.1	2.0	90.3	9.7
2	10	50	109.0	4.3	2.3	86.7	13.3
3	15	100	218.0	5.2	2.4	83.8	16.2
4	30	53	115.5	7.4	2.2	76.6	23.4
5	50	33	71.9	8.1	2.9	71.8	28.2
6	70	28	61.0	9.2	3.1	74.1	25.9
7	85	26	56.1	15.7	2.2	77.6	16.2
8	100	0	0	—	—	—	—

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As we know, the role of MAO is the methylation of metal complexes and the abstraction of one methyl group from complexes. In case of 1, active ion pairs [Y]⁺[RMAO]⁻ are formed by activation of MAO. The increase in the catalyst activity led by addition of TIBA should be attributed to the formation of looser ion pairs. This bulky alkyl aluminum modified MAO could produce bulkier counterions [RMAO]⁻. Moreover, the replacement of alkyl groups from complexes by bulky isobutyl groups also makes ion pairs sterically unfavourable. Both of these effects facilitate the access of monomer to active centres and increase the catalyst activity.^4a,4c,4d,4h

In order to elucidate the influence of [Al_Tot]/[1] molar ratio on isoprene polymerization, polymerization of isoprene catalyzed by 1/MAO/TIBA with a fixed TIBA/Al_Tot (15%) but different [Al_Tot]/[1] were carried out (Table 2). Catalytic activity significantly increased with increasing [Al_Tot]/[1]. The activity at [Al_Tot]/[1] = 150 was 36.8 kg PIP per mol [Y] per h, whereas the activity at [Al_Tot]/[1] = 200 reached 139.8 kg PIP per mol [Y] per h. While the [Al_Tot]/[1] further increased to 500, the activity increased gradually to 245.2 kg PIP per mol [Y] per h. However, it is worth to mention that the polymers obtained at low [Al_Tot]/[1] molar ratios (less than 300) could be dissolved well in common organic solvents (e.g. toluene, chloroform, tetrahydrofuran, and carbon disulfide), whereas the polymer obtained at a high [Al_Tot]/[1] molar ratio (500) contains some insoluble fractions, implying the formation of crosslinking.

Table 2 Polymerization of isoprene with different [Al_Tot]/[1] ratios^a

Entry	AlTot/1	Conv. (%)	Activity (kg PIP per mol [Y] per h)	Mn^b (10^4)	PDI^b	cis-1,4^c (%)	Vinyl-3,4^c (%)
a Conditions: [IP]/[1] = 2400/1, [TIBA]/[AlTot] = 15%, aging at 50 °C for 1 h, polymerization in toluene, 50 °C for 40 min, [IP] = 5 M [AlTot] = [MAO] + [TIBA].b Determined by GPC (THF, PS calibration).c The microstructure was determined by FTIR.
1	70	0	0	—	—	—	—
2	150	15	36.8	16.0	3.3	85.2	14.8
3	200	57	139.8	10.9	2.9	82.8	17.2
4	300	75	183.9	15.1	2.1	77.5	22.5
5	500	100	245.2	17.1	3.7	81.5	18.5

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The catalytic system composed of complex 1 combined with TIBA modified MAO was further applied in polymerization of 1,3-dienes. As shown in Table 3, it seems that polymerization of isoprene and 1,3-butadiene exhibited some controllable characters. The number-average molecular weight (M_n) of PIP produced increased from 5.2 × 10⁴ to 11.9 × 10⁴ with increasing of [IP]/[1] molar ratios from 800 to 2400, while the molecular weight distribution decreased slightly (entry 1–3, Table 3). Using same catalytic system, the increase of [Bd]/[1] molar ratios from 800 to 1600 led to an increase of the M_n of PBd obtained from 18.4 × 10⁴ to 28.2 × 10⁴, and the molecular weight distribution also decreased (entry 4 and 5, Table 3). The random copolymerization of isoprene and 1,3-butadiene with the molar ratio [IP]/[Bd]/[1] = 800/800/1 yielded copolymer with M_n = 24.2 × 10⁴ which approaches to the sum of the M_n of their homopolymers (entry 1 and 4, Table 3).

Table 3 Polymerization and copolymerization of isoprene and butadiene with 1/(MAO + TIBA) system^a

Entry	[IP]/[Bd]/[Y]	Time (min)	Yield (%)	Mn^b (10^4)	PDI^b	cis-1,4^c (%)	Vinyl-^c (%)	trans-1,4^c (%)
a Conditions: [TIBA]/[MAO]/[1] = 15/185/1, aging at 50 °C for 1 h, polymerization in toluene at 50 °C, [IP] = 5 M.b Determined by GPC (THF, PS calibration).c The microstructure was determined by FTIR.d Polymerization in toluene at 25 °C, [Bd] = 3.5 M or [IP] + [Bd] = 3.5 M.
1	800/0/1	15	100	5.2	2.4	83.8	16.2
2	1600/0/1	40	97	8.6	2.3	79.4	20.6
3	2400/0/1	90	95	11.8	2.3	85.1	14.9
4^d	0/800/1	60	100	18.4	2.9	90.0	4.5	5.5
5^d	0/1600/1	60	99	28.2	2.5	85.8	7.3	6.9
6^d	800/800/1	120	95	24.2	3.0

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Kinetic study of the polymerization

Optimal conditions (entry 3, Table 3) were chosen to study the kinetics of the isoprene polymerization catalyzed by complex 1/MAO/TIBA system. As shown in Fig. 2, the value of ln([IP]₀/[IP]) is proportional to the polymerization time, which indicates that the polymerization reaction is first order to the monomer concentration and the number of growing chains may remain constant during the polymerization.

Fig. 2 Plots of ln([IP]₀/[IP]) vs. time for isoprene polymerization catalyzed by 1/MAO/TIBA. Conditions: [IP]/[Al]/[1] = 2400/200/1 [IP] = 5 M.

These samples, yield range from 2 to 91%, were also characterized by GPC. Results are shown in Fig. 3. When yield is low, the molecular weight distribution is distinctly bimodal. With increasing yield, the fraction of the peak of high molecular weight decreases, while the low molecular weight peak grows larger and shifts to higher. When conversion or yield is high enough, the two peaks overlap resulting in an apparently unimodal curve. This phenomenon is consistent with previous works reported by other researchers¹⁵ and us.¹⁶ It indicates that there might be at least two kinds of active species during the polymerization: one was highly reactive but short-lived and produced high molecular weight PIPs corresponding to the higher molecular weight peak at low yield; the other one was lower reactive but kept alive during the polymerization process corresponding to the lower molecular weight peak.

Fig. 3 GPC curves of the PIPs obtained at different yields. Conditions are same to Fig. 2.

To further prove the existence of living polymer chain end, a two-stage polymerization of isoprene was carried out. 800 equiv. of isoprene was added to a completed polymerization solution of isoprene (800 equiv., entry 1, Table 3). Molecular weight of polymers obtained (M_n = 8.7 × 10⁴, PDI = 2.22) is higher than that (M_n = 5.2 × 10⁴, PDI = 2.42) obtained from the initial polymerization solution. Moreover, a peak shift could be clearly recognized from GPC traces (Fig. 4) which demonstrates an increase of molecular weight, and the GPC curve of two-stage polymerization is very close to that of PIP prepared with [IP]/[1] = 1600 molar ratio in one-stage polymerization (M_n = 8.6 × 10⁴, PDI = 2.29, entry 2, Table 3).

Fig. 4 GPC curves of the PIPs obtained from one- or two-stage polymerization. Conditions: one-stage polymerization, [IP]/[Al]/[1] = 1600/200/1, two-stage polymerization, [IP]/[Al]/[1] = (800 + 800)/200/1, other conditions are same to Table 3.

Conclusions

Alkyl yttrium complex supported by carbon-bridged bis(phenolate) ligand was synthesized and characterized by X-ray crystallography. Activated by the combination of MAO and TIBA, the alkyl yttrium complex could efficiently catalyse a polymerization of isoprene and 1,3-butadiene. The effects of TIBA/MAO and the total [Al]/[1] molar ratios on the activity and regioselectivity were investigated. The kinetic studies indicated the polymerization was first order to the monomer concentration but there were multiple active species in the system, which resulted in a relatively board molecular weight distribution.

Experimental

All manipulations of air- and/or moisture-sensitive compounds were performed under argon (Ar) atmosphere using standard Schlenk techniques. Toluene, THF and hexane were dried by refluxing over a benzophenone–sodium mixture and distilled under Ar atmosphere. Isoprene was dried over calcium hydride for 24 h and distilled under Ar atmosphere before use. Polymerization grade 1,3-butadiene was dried by passing through a column filled with activated molecular sieves (4 Å). LiCH₂SiMe₃ was prepared according to a standard procedure.¹⁷ Y(CH₂SiMe₃)₃ was prepared according to the literature.^13a EDBPH₂ and other reagents were commercial available and used without further purification.

Carbon and hydrogen analyses were performed by direct combustion with a Flash EA-1112 instrument. ¹H-NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer. Molecular weight and molecular weight distribution were measured by gel permeation chromatograph (GPC) equipped with a Waters 2414 RI detector and a Waters 1525 isocratic high performance liquid chromatography pump at 40 °C with THF as the eluent at a flow rate of 1.0 mL min⁻¹. The microstructure of polymer was determined by IR spectrometry according the method described in literature,¹⁸ using Vertex-70 FTIR spectrophotometer.

Synthesis of [(EDBP)YCH₂SiMe₃(THF)₃] (1)

EDBPH₂ 1.18 g (2.7 mmol) was dissolved in 10 mL toluene in a Schlenk bottle. The solution was added dropwise to a solution of Y(CH₂SiMe₃)₃ (2.7 mmol) in 50 mL hexane at −30 °C. After stirred at 0 °C for 2 h, the mixture was concentrated to about 10 mL and colourless block crystal formed after stored at −30 °C for several days (1.62 g, 73%). Found C, 67.29; H, 9.16; Y, 10.55; required C, 66.88; H, 9.27; Y, 10.76.

Typical procedure of polymerization

To a previously flame dried ampoule, MAO, TIBA and complex 1 were added and the solution was aged at 50 °C for 1 h before use. Solvents (toluene) and monomers were added to the solution containing catalysts to start the polymerization. Polymerization was carried out at 50 °C and then quenched by adding 10 mL of ethanol after a period of time. The polymer obtained was repeatedly washed with ethanol, cut into small pieces, and finally dried under vacuum at 40 °C to constant weight.

X-ray crystallography

The data collections for the crystals of compound 1 were performed on CrysAlisPro using graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 100 K. The data sets were corrected by empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.¹⁹ All structures were solved by direct methods, and refined by full-matrix least-square methods with the SHELX-97 program package.²⁰ All non-hydrogen atoms were located successfully from Fourier maps and were refined anisotropically.

Crystal data for complex 1. C₉₂H₁₄₉O₁₀Si₂Y₂, M = 1649.11, monoclinic, a = 14.32342(16) Å, b = 19.4258(2) Å, c = 17.3277(2) Å, α = 90.00°, β = 103.2388(12)°, β = 90.00°, V = 4693.19(10) ų, T = 100(2) K, space group P2₁/n, Z = 2, 39 248 reflections measured, 8363 independent reflections (R_int = 0.0386). The final R₁ values were 0.0609 (I > 2σ(I)). The final wR(F²) values were 0.1575 (I > 2σ(I)). The final R₁ values were 0.0702 (all data). The final wR(F²) values were 0.1667 (all data). The goodness of fit on F² was 1.053.

Acknowledgements

The authors gratefully acknowledge the financial supports of the National High Technology Research and Development Program of China (2012AA040305) and the National Natural Science Foundation of China (21174121).

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Footnote

† CCDC 816737. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra06144c

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